Article comprising textured superconductive cuprate material

ABSTRACT

Articles according to the invention comprise a superconductive cuprate (e.g., YBa 2  Cu 3  O 7 ) body containing elongate grains measuring at least about 10 μm along the long axis and having an aspect ratio of at least 10:1. Bodies according to the invention can have relatively high critical current density, as compared to analogous non-textured bodies.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 07/612,286,filed on Nov. 13, 1990, now abandoned which is a divisional ofapplication Ser. No. 07,126,083, filed on Nov. 27, 1987 (now U.S. Pat.No. 5,011,823), which is a continuation-in-part of application Ser. No.07/062,529, filed Jun. 12, 1987, now abandoned.

FIELD OF THE INVENTION

This invention pertains to methods for producing superconductive bodies,and to apparatus and systems comprising a superconductive body producedby such a method.

BACKGROUND OF THE INVENTION

From the discovery of superconductivity in 1911 to the recent past,essentially all known superconducting materials were elemental metals(e.g., Hg, the first known superconductor) or metal alloys orintermetallic compounds (e.g., Nb₃ Ge, probably the material with thehighest transition temperature T_(c) known prior to 1986).

Recently, superconductivity was discovered in a new class of materials,namely, metal oxides. See, for instance, J. G. Bednorz and K A. Muller,Zeitschr. f. Physik B- Condensed Matter, Vol. 64, 189 (1986), whichreports superconductivity in lanthanum barium copper oxide.

The above report stimulated worldwide research activity, which veryquickly resulted in further significant progress. The progress hasresulted, inter alia, to date in the discovery that compositions in theY-Ba-Cu-O system can have superconductive transition temperatures T_(c)above 77K, the boiling temperature of liquid N₂ (see, for instance, M.K. Wu et al, Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908;and P. H. Hor et al, ibid, page 911). Furthermore, it has resulted inthe identification of the material phase that is responsible for theobserved high temperature superconductivity, and in the discovery ofcomposition and processing techniques that result in the formation ofbulk samples of material that can be substantially single phase materialand can have T_(c) above 90 K (see, for instance, R. J. Cava et al,Physical Review Letters, Vol. 58(16), pp. 1676-1679), incorporatedherein by reference.

The excitement in the scientific and technical community that wascreated by the recent advances in superconductivity is at least in partdue to the potentially immense technological impact of the availabilityof materials that are superconducting at temperatures that do notrequire refrigeration with expensive liquid He. Liquid nitrogen isgenerally considered to be one of the most advantageous cryogenicrefrigerants, and attainment of superconductivity at or above liquidnitrogen temperature was a long-sought goal which until very recentlyappeared almost unreachable.

Although this goal has now been attained, there still exist barriersthat have to be overcome before the new "ceramic" superconductors can beeffectively utilized in technological applications. In particular, theceramic high T_(c) superconductive materials are relatively brittle.Development of techniques for fabricating the brittle compounds intobodies of desirable size and shape (e.g., wires or tape), and oftechniques for improving the strength and/or other mechanical propertiesof ceramic superconductive bodies, is an urgent task for the technicalcommunity. Furthermore, techniques for increasing the critical currentdensity J_(c) of bodies formed from superconductive compounds are alsoof great significance.

For a general overview of some potential applications of superconductorssee, for instance, B. B. Schwartz and S. Foner, editors, SuperconductorApplications: SQUIDS and MACHINES, Plenum Press 1977; and S. Foner andB. B. Schwartz, editors, Superconductor Material Science, Metallurgy,Fabrications, and Applications, Plenum Press 1981. Among theapplications are power transmission lines, rotating machinery, andsuperconductive magnets for, e.g., fusion generators, MHD generators,particle accelerators, levitated vehicles, magnetic separation, andenergy storage, as well as junction devices and detectors. It isexpected that many of the above and other applications ofsuperconductivity would materially benefit if high T_(c) superconductivematerial could be used instead of the previously considered relativelylow T_(c) materials.

The art has followed three approaches in producing ceramicsuperconductive compound bodies. One approach comprises providing thedesired compound in powder form, producing a bulk body from the powderby any appropriate technique (e.g., cold or hot pressing in or through adie of desired size and shape, or forming a slurry and producing a tapetherefrom by the doctor blade technique) and heat treating the resultingbody. See U.S. Pat. application Ser. No 368,079, which is a continuationof Ser. No. 036,168, filed Apr. 6, 1987 for E. M. Gyorgy et al, titled"Apparatus Comprising a Ceramic Superconductive Body, and Method forProducing Such a Body, now abandoned." The heat treatment invariablycomprises treatment at a relatively high temperature that is intended toproduce sintering of the powder particles, followed typically byoptimization of the oxygen content of the material. The thus producedsuperconductive bodies typically are relatively porous (e.g., about 85%dense, depending on processing conditions). Furthermore, powderparticles may not always be in intimate contact with their neighbors.The presence of voids and/or poor contact between particles is thoughtto be a possible reason for the relatively low strength and criticalcurrent of bodies produced from superconductive oxide powder by ceramicprocessing techniques.

A recently filed U.S. patent application (Ser. No.426,485, which is acontinuation of application Ser. No. 046,825, filed May 5, 1987 for S.Jin et al, now abandoned) discloses that some properties ofsuperconductive compound bodies (e.g., their mechanical strength) can beimproved by admixture of an appropriate metal powder (e.g., Ag) to thesuperconductive powder.

The second approach typically comprises forming a "preform" byintroducing a quantity of superconductive compound powder into a tubularnormal metal body, reducing the cross section of the preform by, e.g.,drawing through a die (or dies) or rolling, until the desired wire orribbon is produced. The wire or ribbon is then typically wound into acoil or other desired shape, followed by a sintering treatment and,possibly, an oxygen content-optimizing treatment. U.S. Pat. No.4,952,554 and the above referred-to U.S. patent application Ser. No.426,485 disclose techniques for forming metal-clad high T_(c)superconductive bodies. Such bodies typically are also relativelyporous, and have the relatively low J_(c) associated with high T_(c)superconductors produced by ceramic processing techniques.

The third approach to forming superconductive compound bodies comprisesdepositing a thin layer of the superconductive compound on anappropriate substrate. Deposition can be by any appropriate method,e.g., electron beam evaporation, sputtering, or molecular beam epitaxy.Another recently filed U.S. patent application (Ser. No. 126,448, whichis a continuation-in-part of application Ser. No.037,264, filed Apr. 10,1987 for C. E. Rice, now abandoned) discloses that thin superconductivefilms can be produced by forming a solution on a substrate, and heattreating the thus formed thin layer. The high T_(c) compound thin filmsknown to the art are thought to be substantially 100% dense, and atleast in isolated instances relatively high critical currents have beenobserved in such layers.

In view of the fact that technologically significant superconductivewires, ribbons, and other bodies have to be able to carry relativelyhigh current densities and to be able to withstand relatively largeforces, fabrication methods that can result in high T_(c)superconductive bodies having improved properties (including higherJ_(c) and, typically, greater strength and thermal conductivity) wouldbe of considerable significance. This application discloses such amethod.

Definitions

The Ba-cuprate system herein is the class of oxides of nominal generalformula Ba_(2-x) M_(1-y) X_(x+y) Cu₃ O₉₋δ, where M is one of Y, Eu, orLa, and X is one or more optional element different from Ba and M andselected from the elements of atomic number 57-71, Sc, Ca, and Sr.Typically x+y is in the range 0-1 (with Ba and M being at least 50%unsubstituted), and typically 1.5<δ<2.5. In a particular preferredsubclass of the Ba-cuprate system 0≦y≦0.1, with the original X being oneor more of Ca, Sr, Lu and Sc. For further examples see D. W. Murphy etal, Physical Review Letters, Vol. 58(18), pp. 1888-1890(1987).

A slightly different definition of the Ba-cuprate system that has alsobeen used is based on the general formula Ba_(2-y) (M_(1-x) M_(x))_(1+y)Cu₃ O₉₋δ, where M and M' are chosen from Y, Eu, Nd, Sm, Gd, Dy, Ho, Er,Tm, Yb, Lu, La, Sc, Sr or combinations thereof, with typically 0≦x≦1,0≦y≦1, and 1<δ<3. See, for instance, U.S. patent application Ser. No.118,497, titled "Method of Producing Metal Oxide Material, and ofProducing a Superconductive Body Comprising the Material", filed on Nov.9, 1987 for S. Jin, M. Robbins, and R. C. Sherwood, now abandoned.

The La-cuprate system herein is the class of oxides of nominal generalformula La_(2-x) M_(x) CuO₄₋ε, where M is one or more divalent metals(e.g., Ba, Sr, Ca), and x≧0.05, and 0≦ε≦0.5.

A "normal" metal herein is a metal that does not become superconductiveat temperatures of technological interest, typically at temperatures of2 K and above.

A body herein is "relatively dense" if at least a major part of the bodyhas a density that is at least 90% of the theoretical density of thematerial in the part of the body. Preferably the density in the part ofthe body is greater than 95 or even 99% of the theoretical density. Thetheoretical density of Ba₂ YCu₃ O₇ is about 6.4 g/cm³, and the densityof sintered Ba₂ YCu₃ O₇ bodies (heat treated to optimize thesuperconductive properties) typically is no more than about 5.5 g/m³(about 85% of theoretical).

Summary of the Invention

We have discovered that superconductive compound (e.g., Ba-cuprate andLa-cuprate) bodies can be produced by a process that comprises melting aquantity of precursor material, and cooling at least a portion of themelt such that a solid body of a desired shape (e.g., a filament)results, with at least a substantial portion of the superconductive bodyhaving a density greater than about 90% of the theoretical density ofthe superconductive compound. The inventive method typically furthercomprises heat treating the solid body in an oxygen-containingatmosphere so as to impart the desire superconductive properties to thebody. For instance, at least for an exemplary member of the Ba-cupratesystem (nominal composition Ba₂ YCu₃ 0₇) an appropriate exemplary heattreatment comprises maintaining the body at a temperature in theapproximate range 850-950° C. in an oxygen-containing atmosphere for aperiod of time in the range 1-48 hours, followed by slow cooling.Although not a requirement, in many cases it will be desirable for theinventive method to be carried out such that the resulting body consistssubstantially of single phase material.

The inventive technique represents a complete departure from prior artprocessing of bulk Ba-cuprate and La-cuprate material. These materials,which are ceramics, have in the past been processed by ceramictechniques. In general, ceramic processing techniques do involve hightemperature treatment (e.g., sintering). However, standard hightemperature processing of ceramic materials is carried out attemperatures below the melting point of the material, and melting ofceramic material is, to the best of our knowledge, not used in theproduction of any commercially significant ceramic.

All the currently known high T_(c) ceramic superconductive compounds areeither La-cuprates or Ba-cuprates. Both these systems have complicatedphase diagrams, with single phase-based superconductivity occurring onlyin relatively narrow compositional ranges. Due to these circumstancesconventional theory suggests that solidification of material from a meltof composition corresponding to that of the superconductive phase willresult in decomposition into a multiphase material that is notsuperconductive, or only partly superconductive at "high" temperatures(e.g., at or above 77 K).

Thus, not only is processing that involves melting of the ceramicmaterial not within the normal repertoire of ceramicists but, based onthe phase diagram of the prototypical Ba-cuprate YBa₂ Cu₃ 0₉₋δ), a manskilled in the art as good reason for not melt-processing the material.

However, we have made the unexpected discovery that it is possible toproduce superconductive material by a method that comprises cooling fromthe single phase liquid region (exemplarily sufficiently rapid coolingsuch that phase separation is substantially avoided or minimize), orthat comprises cooling from the solid+liquid region of the phasediagram.

After solidification of the melt a variety of treatments may be applied.For instance, the sample can be maintained in 0₂ at a temperaturebetween the solidus and that of any solid state transformation (if sucha transformation exists) for a relatively long period (e.g., 1-24 hrs)to facilitate homogenization and/or grain growth, followed by a slowcool (possibly with intermediate soaks) in O₂ to room temperature. Onthe other hand, the solidified sample can be cooled slowly (typically inO₂) to room temperature, with a later homogenization treatment in O₂ ata temperature relatively close to but below the solidus (exemplarily1-24 hours at 850-950° C.). Intermediate treatment schedules are alsopossible.

In a currently preferred embodiment the melt is cooled relativelyrapidly (exemplarily within about 1-600 seconds) from the liquid regionof the phase diagram to an intermediate temperature slightly below thesolid+liquid region (exemplarily 10-100° C. below the solidus and aboveany solid state phase transition temperature that may be present in thesystem), followed by a heat treatment that favors the growth ofcrystallites of the superconductive phase and avoids thermal shock andthe consequent formation of microcracks. If the cooling from the melt iscarried out too slowly (e.g., within more than about 10 minutes) anunacceptable amount of phase separation (typically more than about 15%by volume of non-superconductive phase) is likely to occur, and if thecooling is carried out too quickly (e.g., within less than about 1second), microcracks may form. The details obviously will depend, interalia, on the amount and shape of the material that is to be solidified;The heat treatment of the solidified material exemplarily comprises aslow cool (e.g., furnace cool, 1-100 hours) in O₂ to about roomtemperature, and optionally comprises a soak in O₂ at the intermediatetemperature, or some other elevated temperature. Exemplarily, a melt ofcomposition YBa₂ Cu₃ O_(x) (x˜7) is maintained under O₂ at about 1300°C. for about 5 minutes, the melt is then rapidly cooled in O₂ to about950° C., followed by a furnace cool in O₂ to room temperature. Theresulting material typically has spherulitic microstructure, with manygrains being oblong, having a long axis of about 20-200μm. The materialtypically is substantially single phase, essentially 100% dense, and hasT_(c) of about 92 K and J_(c) of more than about 2000 A/cm² (at 77 K,with H=0).

In a further exemplary embodiment the material is heated to atemperature in the solid+liquid region of the phase diagram, optionallymaintained in that region under O₂ for a period sufficient to result inestablishment of approximate phase equilibrium (e.g., 5 second-5 hours),followed by a relatively slow cool (e.g., furnace cool 1-24 hours) in O₂to about room temperature (with an optional soak or soaks at one or moretemperatures below the solidus not excluded). Exemplarily, a melt ofcomposition YBa₂ Cu₃ O_(x) is maintained at 1030° C. for about 1 hourand furnace cooled The resulting material typically has predominantlyspherulitic microstructure, comprises, in addition to thesuperconducting phase, crystallites of nominal composition Y₂ BaCu₃ O₅and also copper oxide and barium oxide. The material typically isessentially 100% dense, and has exemplarily T_(c) of about 92 K andJ_(c) of about 1700 A/cm² (at 77 K, with H=0).

As is apparent from the above cited properties, superconductive bulksamples produced according to the invention can have higher J_(c)(including substantially greater J_(c) in a magnetic field) than priorart bodies. Furthermore, bodies produced according to the inventiontypically also can have greater mechanical strength and greater thermalconductivity.

The known superconductive oxides are relatively reactive and can beexpected to interact with most common crucible materials. Consequently,it may be advantageous to use a crucible-free method of melting of thestarting material, or to melt the starting material in a crucible thatprovides a particular constituent to a starting material that isinitially deficient in that constituent. For instance, Y-poor Ba-Y-Cuoxide starting material can be melted in a Y₂ 0₃ -lined crucible, withthe starting composition, melt temperature, soak time, etc. chosen suchthat the solidified material has the desired 1:2:3 ratio of Y:Ba:Cu.

The inventive, method, or obvious variations thereof, can be used inconjunction with the known superconductive compounds, namely, themembers of the Ba-cuprate system and of the La-cuprate system. Therehave been reports that, for instance, in some samples of nominalcomposition Ba₂ Y₁ Cu₃ O₆.9 indications of superconductivity weredetected at temperatures above 100 K If these reports are correct, thenit is likely that an unidentified phase of the material becomessuperconductive at a temperature above 100 K. We consider it likely thatthe inventive method, or an appropriate extension thereof, could be usedto produce superconductive bodies that comprise the alleged high T_(c)phase, should the phase exist. Furthermore, the inventive method, or anappropriate extension thereof may even likely be useful in connectionwith making superconductive bodies from non-cuprate superconductivecompounds (e.g., nitrides, sulfides, hydrides, carbides, fluorides, andchlorides), should such non-cuprate superconductive compounds exist.However, in the remainder of this application we will generally onlyrefer to cuprate superconductors, in particular, to the Ba-cuprate ofnominal composition YBa₂ Cu₃ O₇. This is for ease of exposition only,and does not imply a limitation of the inventive method to that system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic phase diagram, depicting a cross section of atentative ternary phase diagram of the Y₂ O₃ -BaO-CuO system;

FIG. 2 schematically depicts the cross section of an exemplary wireaccording to the invention;

FIG. 3 schematically shows a further exemplary embodiment of theinvention; and

FIG. 4 schematically depicts exemplary apparatus according to theinvention, namely, a superconducting solenoid

FIGS. 5 and 6 show photomicrographs of a superconductor prepared by theprior art technique and prepared according to the invention,respectively;

FIGS. 7 and 8 similarly show a scanning electron fractograph of priorart superconductive material and material prepared according to theinvention, respectively;

FIGS. 9, 10 and 11 show a low magnification photomicrograph and two highmagnification micrographs, respectively, of a further superconductivesample according to the invention; and

FIG. 12 represents a collection of data on J_(c) as a function ofmagnetic field, for YBa₂ Cu₃ O₉₋δ in a variety of (prior art) forms, andalso produced according to the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

Tentative ternary phase diagrams of the Y₂ 0₃ -BaO-CuO system haverecently been reported. See, for instance, K. G. Frase et al, "PhaseCompatibilities in the System Y₂ 0₃ -BaC-CuO at 950° C.," submitted Apr.7, 1987 to Communications of the American Ceramic Society. FIG. 1 showsa tentative partial schematic phase diagram, derived from theabove-referred to ternary phase diagram, that illustrates some aspectsof the invention. As will be understood by those skilled in the art, thephase diagram is a schematic cross section of the ternary diagram alongthe tie-line of non-superconducting BaY₂ CuO₅ and superconducting Ba₂YCu₃ O₇. It will also be appreciated that further research may requiremodification of the phase diagram.

Compositions in fields 10, 11 and 12 of FIG. 1 are mixed solid phases(consisting of BaY₂ CuO₅ /Ba₂ YCu₃ O₇ and Ba₂ YCu₃ O₇ /BaCuO₂ /CuO,respectively. Compositions in field 13 are liquid+solid (the solid beingBaY₂ CuO₅). Finally, compositions in field 14 are single phase liquid.As is well known, the presently known high T_(c) superconductive phasein the Ba-Y-Cu-O system has nominal composition Ba₂ YCu₃ O₇. (Theoptimal oxygen content is not necessarily equal to 7 but may differslightly therefrom, e.g., 6.9. We intend to include such minordepartures within the above nominal composition.)

Heating a sample of material of composition Ba₂ YCu₃ O₇ above thesolidus line 17 (about 980° C. in air or about 1010° C. in oxygen)results in (partial or complete) melting. For instance, if the sample isheated to a temperature in field 13 of FIG. 1 (e.g., corresponding topoint 15) then partial melting occurs, with the equilibrium compositionsof the liquid and solid portions determined by the familiar lever rule.On the other hand, the equilibrium phase in field 14 of FIG. 1 is auniform liquid. However, relatively slow heating through field 13produces enrichment of the solid phase with Y, with attendantundesirable increase in the melting temperature. Consequently, in atleast some of the embodiments of the invention it is consideredadvantageous to raise the temperature of the starting materialrelatively rapidly at least through field 13 to a temperature in field14 (e.g., point 16). Exemplarily, the temperature is raised such that asample of Ba-cuprate spends less than about 5 minutes (preferably lessthan 2 or even 0.5 minutes) in field 13 (or an equivalent solid-liquidregion of an applicable phase diagram). In some cases it may beadvantageous to permit establishment of thermal equilibrium by soakingthe sample at a temperature close to but below the solidus, followed byrapid heating to the liquid region.

After being maintained in region 14 for a relatively non-critical period(typically long enough to ensure homogeneity of the melt and shortenough to avoid undesired uptake [or possibly loss] of material by[from] the melt) at least a portion of the melt is rapidly cooledthrough field 13 (or an equivalent solid+liquid region of an applicablephase diagram) to a temperature (e.g., 18) below the solidus but abovethe orthorhombic/tetragonal transition temperature of YBa₂ Cu₃ O_(x) atabout 700° C. The cooling rate should be such that phase separation issubstantially avoided or minimized. Exemplarily, the cooling ratethrough field 13 for a sample of Ba-cuprate is typically greater thanabout 100° C./min, and preferably greater than 200° C./min.

In many cases, it will be desirable to alter the shape of all or part ofthe molten material prior to its solidification, e.g., by drawing afiber or a ribbon from the melt, by casting into a mold, or by extrusionthrough a die. The details of the shaping step as well as of the rapidcooling step depend, inter alia, on the desired shape of thesuperconductive body. For instance, a fiber or ribbon may be drawn orextruded by techniques of the type employed in the manufacture ofoptical fibers or of rapidly quenched glassy metals. The casting of bulkbodies will typically require cooling of the mold or die. The shape ofthe solidified body need not necessarily be the final shape of thesuperconductor, and at least in some cases it may be advantageous tocarry out a separate solid state shaping step, e.g., hot forming at atemperature close to the solidus.

Although currently not preferred, it may at times be advantageous tocarry out the shaping of the superconductive material under conditionssuch that the material is in the solid+liquid region (field 13 of FIG.1). In particular, the conditions can be chosen such that thesolid+liquid mixture has an appropriate viscosity that allows easiershaping of the material. As will be readily appreciated, the processingtime in the solid+liquid two phase region advantageously is kept to aminimum (exemplarily less than 10 minutes, preferably less than 1minute) to reduce compositional separation. As discussed above,subsequent heat treatment should produce substantial homogenization.

Subsequent to the shaping and rapid solidification the body is typicallyheat treated so as to obtain material of the appropriate composition(e.g., oxygen content) and crystal structure and, possibly, tohomogenize the material. Exemplarily, bodies made from the currentlypreferred Ba-cuprate (nominally composition Ba₂ YCu₃)₆.9) can optionallybe heat treated in oxygen (or an oxygen-containing atmosphere such asair) at a temperature in the range 850-950° C. for a period in range1-48 hours, typically followed by relatively slow cooling (e.g., averagerate <100° C./hour) in oxygen (or an oxygen-containing atmosphere to atemperature below about 300° C., so as to avoid formation ofmicrocracks. Maintaining the sample at an intermediate temperature forsome period (e.g., 10 minutes to 15 hours) may at times be advantageous,e.g., to facilitate attainment the optimal oxygen content

As will be appreciated, the details of the heat treatment will depend,inter alia, on the shape of the body. For instance, resolidified bodieshaving one or two relatively small dimensions (e.g., ribbons and fiber,respectively) may not require as extended post-solidification heattreatment as bulk bodies, since the diffusion distances (e.g., for O₂)are much smaller in the former cases than in the latter. Thus, it may bepossible to carry out the heat treatment of fiber and/or ribbon as acontinuous in-line process right after formation of the fiber or ribbon.

FIGS. 5 and 7 show a photomicrograph and a scanning electron fractographof prior art sintered ceramic high T_(c) superconductive material(nominal composition YBa₂ Cu₃ O₆.9, having T_(c) of about 92 K and J_(c)of about 700 A/cm² at 77 K and H=O). As can be seen from the Figures,the prior art material is granular (average grain size substantiallyless than 10 μm) and porous, essentially without texture. FIGS. 6 and 8show a corresponding micrograph and fractograph, respectively, ofsuperconductive material produced according to the invention. Thenominal composition of the material of FIGS. 6 and 8 is the same as thatof the sintered material of FIGS. 5 and 7. The inventive material alsohas T_(c) of about 92 K but has J_(c) of about 3000 A/cm² at 77 K andH=0. The material according to the invention was produced by a processthat comprises rapidly (within less than about 5 seconds) cooling themelt from about 1300° C. to about 950° C., followed by a furnace cool toroom temperature. As can be seen from FIGS. 6 and 8, the inventiveprocess results in essentially 100% dense material that is stronglytextured, with a substantial portion being relatively large elongatecrystallites (e.g., having a long dimension that is typically greaterthan about 10 μm, with aspect ratio typically 10:1 or greater, and withthe long axis tending to lie in the basal plane of the orthorhombicsuperconductor), and that is predominantly single phase. The texture ofthe exemplary material is spherulitic.

However, by providing for directional cooling macroscopically orientedgrowth can be obtained. Oriented growth (including spherulitic growth)typically results in a structure in which neighboring crystallites havesimilar orientations, with relatively low angle boundaries betweenadjacent crystallites. This is considered to be of significance in thislayered material which has anisotropic superconductivity and thermalexpansion, and may be an aspect that contributes to the improved J_(c)of material produced according to the invention, as compared to priorart sintered material.

A further embodiment of the inventive method is exemplified by FIGS. 9,10 and 11, which show respectively a low and two high magnificationphotomicrographs of high T_(c) superconductive material producedaccording to the invention. The material was produced by maintaining apressed and sintered sample of the staring material (powder ofcomposition YBa₂ Cu₃ O_(x)) in the solid+liquid region (13 of FIG. 1) ofthe phase diagram (1 hour at 1030° C.), followed by a furnace cool toroom temperature, all under O₂. The resulting material had T_(c) ofabout 92 K, and J_(c) of about 1700 A/cm² at 77 K and H=O.

In FIGS. 10 and 11 the needle-shaped crystallites are superconductiveYBa₂ Cu₃ O₇, the rounded crystallites are non-superconducting Y₂ BaCuO₅,and the bright phase is non-superconducting CuO(+BaO). The material thusis clearly not single phase (although more than 80% by volume typicallyis superconductive material), but the superconductive phase typicallyforms a continuous network As FIG. 9 shows, the non-superconductive"bright" phase is non-homogeneously distributed, being concentratedmainly in thin regions between relatively large regions that arerelatively free of the bright phase. FIG. 10 depicts a region thatincludes a "boundary" between two of the referred to large regions ofFIG. 9, and FIG. 11 depicts a portion of the interior of one of thelarge regions.

FIG. 12 gives exemplary data of critical current density as a functionof applied magnetic field, for samples of nominal composition YBa₂ Cu₃O₇ produced by a variety of techniques. Line 120 represents bulk samplesproduced according to the invention. Prior art bulk samples (i.e.,sintered) typically fall into region 121. The improvement in J_(c)including the slower decrease of J_(c) with increasing magnetic field,is apparent from a comparison of 120 and 121. Line 122 pertains tosingle crystal samples. Line 123 pertains to thin film results. Furtherthin films have been found to have J_(c) (H=O) in the region indicatedby bracket 124.

As FIG. 12 demonstrates, there exists a very large difference in J_(c)between polycrystalline bulk samples (especially prior art material,i.e., material with randomly oriented small grains) and single crystalsand thin films. It is widely believed that this difference is due, atleast in significant part, to grain boundary resistance effects. Sucheffects could be caused, inter alia, by the presence of inhomogeneitiesor impurities, mechanical defects (e.g., stress concentration ormicrocracks), altered stoichiometry (e.g., oxygen content), structuraldeviation, or crystal orientation change at grain boundaries. Thepresence of voids may also adversely affect the transport properties.These and/or possibly other effects are likely to result in low J_(c)regions at grain boundaries separating high J_(c) gains. Thus atreatment that eliminates or reduces these low J_(c) regions or perhapsincreases the current carrying capacity of these regions could result inincreased J_(c) of a bulk sample. We believe that the improved J_(c)observed in samples produced according to the invention is due to suchan effect. In particular, the larger grain size and, significantly, thehigh degree of texture (with the attendant reduction in averageorientation change at grain boundaries) are thought to at leastcontribute to the observed improvement. Other, so far unidentifiedfacts, may of course also be contributing to the improvement

The superconductive bodies produced according to the invention may becoated (e.g., for purpose of electrical or thermal stabilization, ormechanical or environmental protection) with a suitable material, e.g.,a normal metal such as Ag, Cu, Zn, In, Cd, Al, Sn, etc. The coating canbe applied by any suitable process, e.g., by evaporation, or by dippingin the molten metal. The bodies may also be coated with insulatingmaterial (e.g., polymers or some oxides), either alone or on top of ametal coating. FIG. 2 schematically depicts in cross section anexemplary coated wire according to the invention, wherein 21 is themelted and resolidified high T_(c) ceramic superconductive material, and22 is cladding material, exemplarily a metal cladding.

It is also envisaged to form a "wire" by a process substantially asdescribed in the above referred to U.S. Pat. No. 4,952,554 (incorporatedherein by reference), except that the cladding material is a relativelyductile high melting point metal such as stainless steel Nb or Ta(including a cladding consisting of ductile high melting point firstmetal matrix with second metal particles embedded therein, the secondmetal also having a high melting point and having substantiallydifferent chemical behavior than the first metal, susch that the secondmetal particles can be removed by selective etching), with typically abarrier layer on the inside of the cladding. After drawing the wire froma preform and, typically, shaping the wire such that it is at leastapproximately in its final form, the wire is heated (either all of itsimultaneously or consecutively, e.g., by passing a hot zone along thelength of wire) such that the oxide powder core of the wire melts,followed by resolidification and heat treatment.

In one exemplary embodiment the cladding is hermetically sealed suchthat all the O₂ that was given off by the oxide during melting and hightemperature treatment is still available to re-oxidize the material atlower temperatures. In another exemplary embodiment O₂ is pumped intothe wire from the wire ends through the voids that resulted from thedensification of the oxide material upon melting and resolidification.And in still another exemplary embodiment the wire is made porous byselective etching of the second metal particles, and O₂ is supplied tothe oxide through the porous cladding during the heat treatment.

The invention can be practiced in a number of ways. For instance, thematerial that is to be melted (the "charge") can be a sintered body orcompressed powder pellet of superconductive oxide produced by aconventional technique, or the charge can be a mixture (in appropriateratios) of the starting materials for oxide production (e.g., BaCO₃, Y₂O₃, and CuO powder, or nitrate, acetates, oxalates, etc., containing thedesired metals). The composition of the charge can be such that therelevant metals (e.g., Y, Ba, and Cu) are present in the same ratio asthey are found in the relative superconductive phase (e.g., 1:2:3), orthe melt can be deficient in one (or more) of the metals, with thedeficient metal(s) to be augmented in the melt. For instance,Y-deficient starting materials can be melted in a Y₂ O₃ -lined crucible,with Y being augmented from the crucible liner. Such a proceduretypically will reduce the likelihood of poisoning of the melt.Furthermore, material once melted according to the invention can bere-melted, for instance, to facilitate further shaping or to eliminatetrapped bubbles and/or other inhomogeneities. In some instances it maybe advantageous to heat the charge slowly to a temperature below theliquidus (or to maintain the charge at that temperature for someperiod), followed by rapid heating into (and preferably through) thesolid+liquid region. Such treatment may reduce oxygen evolution in theliquid material.

The charge can be melted in a crucible, contact-free (e.g., by torch,RF, electrical heater, or laser melting the end or middle of a suspendedrod of the starting material), or such that the molten charge is incontact only with other charge material (e.g., by forming a moltenpuddle in a quantity of the starting material). Furthermore, a core of afirst material (e.g., silver wire) can be coated with superconductivematerial (e.g., Ba₂ YCu₃ O₇ powder mixed with a binder, and possiblyalso with a metal powder such as Ag powder, the presence of the lattercan improve the adhesion of the ceramic to the metal core. See the abovereferred to U.S. patent application Ser. No. 426,485). The coated coreis then moved through a heating zone such that the binder is driven offand at least the outer portion of the superconductive material coatingis melted and rapidly resolidified, without substantial melting of thefirst material core. This can be accomplished by a variety oftechniques, such as by means of circumferentially arranged lasers orelectron beams, by means of a well controlled ring burner, or bymicrowave heating. FIG. 3 schematically depicts, in cross-sectionalview, an exemplary body of the above described type, wherein 31 is thecore (e.g., silver wire), 32 is the ceramic superconductive shell, withmaterial outside of line 33 being resolidified (relatively dense)material, and material inside of 33 being less dense material, forinstance, being in the sintered state. Layer 34 is a coating(exemplarily a polymer coating). It will be appreciated that 32 could inprinciple be completely resolidified superconductive material. Theinvention can also be practiced by forming a layer of precursor material(e.g., a YBa₂ Cu₃ O₇ layer produced by a prior art technique) on asubstrate (e.g., ZrO₂), melting all or a portion of the precursormaterial, rapidly cooling the melted material below the solidus,followed by an appropriate heat treatment. The melting can be carriedout by any appropriate means, including laser melting or by means of aheat lamp.

Another exemplary embodiment of the invention comprises castingsuperconductive slit lamellae of the type used in Bitter magnets,assembling a stack of lamellae, with appropriate insulator materialbetween neighboring lamellae and with the lamellae arranged that anygiven lamella overlaps with its neighboring lamella or lamellae (therebyforming a continuous spiral), melting at least the overlap regions orotherwise establishing superconductive contact between the overlapregions, and heat treating the thus produced Bitter magnet as disclosedherein.

A toroidal superconductive magnet can also be formed by casting of themolten oxide into a tubular mold.

The inventive method typically can be used to fabricate high T_(c)superconductive bodies that are relatively dense throughoutsubstantially all (typically>95%) of the resolidified portion of thebody. Bodies according to the invention typically have substantiallygreater fracture toughness than less dense identically shaped bodies ofthe same composition, fabricated by a prior art technique such assintering of pressed powder. The latter frequently has densities in therange 70-85% of the theoretical density, depending on the heat treatmentThe improvement in fracture toughness typically is at least about 50%.Inventive bodies typically also have critical currents that aresubstantially larger (exemplarily at least about 20% larger) than thoseof identically shaped prior art superconductive bodies of the samecomposition. The transition temperature T_(c) of bodies according to theinvention can be identical to that obtainable in prior art bodies of thesame composition. Some treatment conditions may lead to a slightlybroadened transition, and materials produced according to the inventionand having a somewhat broadened superconductive transition (but stillreaching R=0 at or above 77 K) are also contemplated.

Bodies according to the invention can be advantageously employed in avariety of apparatus and systems, e.g., those discussed in thepreviously referred to books by S. Foner and B. B. Schwartz. Exemplaryof such applications is the superconductive solenoid schematicallydepicted in FIG. 4, wherein 41 is a clad superconductive wire accordingto the invention, and 42 is a tubular body that supports 41.

EXAMPLE I

Powder of nominal composition Ba₂ YCu₃ O₇ and approximately 5 μm averagediameter was prepared in a conventional manner (see, for instance, R. J.Cava et al, op. cit.), and pressed into a 3.1×3.1×31 mm pellet. Thepellet was heated such that its central portion melted. Melting anddrawing apart of the solid end portions resulted in formation of anelongated necked-down region whose minimum diameter was about 1.25 mm.After rapid solidification of the molten portion the sample was heattreated in oxygen (16 hours at 900° C.; furnace cooled to 600° C.; 2hours at 600° C.; furnace cooled to 250° C.). The solidified portion ofthe sample was found to be superconductive, with transition onsettemperature of about 98 K and completion (R=0) at 92 K. The normalizedmagnetic susceptibility transition behavior of the resolidified materialwas essentially the same as that of an unmelted comparison sample ofidentical composition. In particular, the superconductive transition inboth samples was substantially complete at 90 K.

EXAMPLE II

Ba-cuprate powder as described in Example I was mixed with 17% by volumeof Ag powder (1.3 μm average diameter). To the mixture was added 50%b.v. of a commercial acrylic binder (Cladan No. 73140 obtained fromCladan, Inc., San Marcos, Calif.) dissolved in 1,1,1 trichloroethane. A0.25 mm diameter Ag wire was dipped into the thus produced slurry. Thecoated wire was heat treated for 16 hours at 900° C. in O₂, followed byfurnace cooling to room temperature. The resulting wire preform had adiameter of about 0.75 mm, a superconductive transition temperature(R=0) of about 92 K, and a critical current density of about 100 A/cm²at 77 K. The preform wire was then heated so that at least the outerportion of the ceramic coating was melted rapidly while substantiallymaintaining the geometry, followed by rapid cooling andresolidification. The wire was then heat treated substantially asdescribed in Example I. The thus produced wire had a substantially 100%dense outer ceramic layer that adhered well to the Ag core, T_(c) (R=0)of 91 K, and a critical current density of about 400 A/cm².

EXAMPLE III

A ceramic-coated wire was produced substantially as described in ExampleII. The wire was then dipped into molten In, resulting in formation of a0.125 mm thick In coating. The wire was superconducting, with T_(c) ofabout 92 K.

EXAMPLE IV

A sintered pellet of Ba₂ YCu₃ O₇ powder, produced in the conventionalmanner, was rapidly heated above the liquidus, and a drop (approximately6 mm diameter) of the melt caused to fall on a 40° inclined steel plate.At the moment of impact the drop was flattened into ribbon shape bydropping a steel block onto it. The contacting surface of the steelblock carried a grooved pattern (1.25 mm pitch, 0.375 mm depth). Theresulting patterned ribbon was about 0.75 mm thick. The ribbon was heattreated for 16 hours at 900° C., furnace cooled to 600° C., maintainedfor 2 hours at 600° C., then furnace cooled to 200° C., all in O₂. Theresulting ribbon had T_(c) (R=0) of about 92 K, and J_(c) of about 200A/cm² at 77 K. It was substantially 100% dense, appeared to beessentially single phase, and its fracture toughness is at least 50%greater than that of a sintered test body of identical shape andcomposition.

EXAMPLE V

A rod is produced from Ba₂ YCu₃ O₇ powder by pressing in a conventionalmanner. The rod is heated such that a puddle of molten material isformed on the upper end of the rod, the melt is contacted with asilver-coated stainless steel bait rod wire, and the bait rod iswithdrawn at a rate such that a 0.125 mm diameter fiber of the ceramiccompound is continuously formed. The thus formed substantially 100%dense ceramic fiber is wound on a 1 m diameter spool with Ag-coatedsurface, and heat treated on the spool substantially as described inExample I. The heat treated fiber is coated by drawing it through moltenCd. The coated fiber has T_(c) of about 93 K, and is wound on a tubularmandrel (having 1 m outer diameter), thereby producing a superconductingsolenoid.

EXAMPLE VI

A substantially 100% dense ribbon (0.125×1.25 mm cross section) ofnominal composition Ba₂ YCu₃ O₇ is formed in continuous manner by meltspinning, i.e., by causing a continuous stream of the molten material tofall onto a spinning ceramic-coated wheel maintained at about 400° C.After heat treatment substantially as described in Example I the ribbonis substantially single phase material and has T_(c) of about 92 K.

EXAMPLE VII

Powder as described in Example I was pressed into a 2×2×30 mm pellet,the single phase pellet heated to 950° C., followed by rapid (about 200°C./min) heating to about 1300° C., and held at that temperature forabout 2 minutes. The resulting single phase liquid was then rapidlycooled (about 200° C./min) to about 950° C., held at that temperaturefor 20 minutes, followed by furnace cooling to room temperature. All ofthe heat treatment was carried out in O₂ at ambient pressure. The samplewas then given a homogenization and oxygen adjustment treatment asfollows: heated to 920° C., soaked for 16 hours, cooled to 600° C. at100° C./hour, cooled to below 300° C. at 20° C./hour, all in O₂ atambient pressure. The sample had T_(c) (R=0) of 93 K, and J_(c) (77 K)at H=0, 50, 100, 200, and 10,000 gauss of 3100, 2300, 1320, 570, and 130A/cm², respectively.

A prior art sample of identical composition and geometry (sintered at920° C. for 16 hours, furnace cooled to room temperature, all in O₂) hadT_(c) (R=0) of 92 K, and J_(c) (77 K) at 0, 50, 200, and 10,000 gauss of570, 130, 20, and 3 A/cm², respectively.

EXAMPLE VIII

A pellet as in Example VII was heated to 1030° C., maintained at thattemperature for 2 hours, and furnace cooled to room temperature, all inO₂ . Subsequently, the sample was heated to 920° C., maintained at thattemperature for 16 hours, followed by a furnace cool to roomtemperature, all in O₂. The material had T_(c) (R=0) of 93 K, and J_(c)(77 K) at-0, 50, 200, and 10,000 gauss of 1700, 1210, 310, and 100A/cm², respectively

EXAMPLE IX

A sample was prepared substantially as in Example VII, except that thepellet was maintained at 1030° C. for only 20 minutes, followed by arapid cool to 950° C., followed by a furnace cool to room temperature.The sample had T_(c) (R=0) of 91 K and J_(c) (77 K) at 0, 200, and10,000 gauss of 1600, 380, and 120 A/cm² , respectively.

EXAMPLE X

A sample was prepared substantially as described in Example VII, exceptthat the pellet was held at 1300° C. for 5 minutes, then transferredrapidly (about 0.5 seconds) into a 980° C. region of the furnace, andone end of the pellet exposed to a blast of O₂ , leading to rapidcooling with directional solidification. The sample had T_(c) (R=0) of92 K and J_(c) (77 K, H=0) of 7400 A/cm².

EXAMPLE XI

A 0.075 mm thick film of nominal composition YBa₂ Cu₃ O₇ is formed on aZrO₂ substrate by coating a surface of the substrate with a slurry(containing about 30% by volume binder as in Example II, the remainderbeing YBa₂ Cu₃ O_(x) powder), heating (50° C./hour) the coated substrateto 900° C. to remove the binder, then raising the temperate to 950° C.,maintaining it at that temperature for 2 hours, then rapidly (about 500°C./min) heating the precursor material by means of a heat lamp to 1300°C., maintaining this temperature for 10 seconds, then rapidly (200°C./min) cooling the melted precursor material to 950° C. The substratewith the solidified layer thereon is maintained at 950° C. for 1 hour,then is slowly (30° C./hour) cooled to room temperature. All of the heattreatment is carried out in O₂ at ambient pressure. The thus producedsuperconductive film has T_(c) of 92 K, and J_(c) (H=0) in excess of1000 A/cm².

What is claimed is:
 1. An article comprising a body that comprises bulksuperconductive cuprate;CHARACTERIZED IN THAT the bulk cuprate compriseselongate grains, associated with a given elongate grain being a longaxis, an aspect ratio, and a crystal structure including a basal plane,at least a substantial fraction of the elongate grains measuring atleast about 10 μm along the long axis and having an aspect ratio of atleast 10:1, with the long axis substantially lying in the basal plane,the aspect ratio being the ratio between the long and short graindimension in the basal plane, with a density of at least a substantialportion of the bulk cuprate being greater than 90% of the theoreticaldensity of the cuprate, and wherein the cuprate is of nominalcomposition YBa₂ Cu₃ O₇.
 2. Article according to claim 1, wherein saidbody further comprises silver in contact with the superconductivecuprate.
 3. Article according to claim 2, wherein silver is dispersedwithin the body.
 4. Article according to claim 1, wherein the bodycomprises a layer of superconductive cuprate disposed on a substrate. 5.Article according to claim 1, wherein the body comprises an elongatecore containing the superconductive cuprate and optionally containingsilver, and wherein the elongate core is surrounded by a metal coating.6. Article according to claim 1, wherein the article comprises asuperconductive magnet.
 7. Article according to claim 1 wherein, duringnormal use of the article, at least a portion of said body is exposed toa magnetic field.
 8. Article according to claim 1 wherein, during normaluse of the article, an electrical current flows through said body. 9.Article according to claim 1, wherein in a given portion of said bodythe elongate grains have similar orientation, with the long axis of agiven elongate grain pointing in substantially the same direction as thelong axes of the elongate grains adjacent to the given grain.